Vinyl chloride homopolymers and copolymers are produced commercially by suspension, emulsion, solution, and bulk techniques, which yield products that differ widely in such properties as particle size and shape, porosity, total surface area, molecular weight, and purity. The end-use applications in which the polymers can be used are determined to a large extent by the procedure by which they wee prepared.
Suspension polymerization is the most widely used of the polymerization procedures for vinyl chloride because it yields products that have combinations of properties that make them adaptable to the dry blending, preblending, and thermoplastic processing techniques that are used in the fabrication of vinyl chloride polymer products. In addition, suspension polymerization processes have economic and control advantages over the other types of polymerization processes.
In the suspension polymerization processes, a monomer component that comprises vinyl chloride is dispersed in an aqueous medium using vigorous agitation. Polymerization is started in the monomer droplets by means of a monomer-soluble initiator. A surfactant is used to minimize coalescence of the growing polymer particles by forming a protective layer around them. The polymerization is carried out at a temperature and a pressure that will yield a polymer having the desired molecular weight. The product of the polymerization is an aqueous slurry that contains 5% to 50% by weight, usually 20% to 40% by weight, of a vinyl chloride polymer in the form of particles that range in size from about 25 microns to about 500 microns, not more than 0.1% by weight of the surfactant, and up to 5% by weight of unreacted vinyl chloride. Most of the unreacted monomer is removed by heating the slurry under reduced presure to about 65.degree. C. As it is ordinarily practiced, this stripping procedure reduces the monomer content of the slurry to about 1000 parts to 15,000 parts by weight of vinyl chloride per million parts by weight of polymer in the slurry. Further processing yields dry products that may contain 500 ppm or more monomer.
In view of the present safety standards that require that the amount of monomeric vinyl chloride in vinyl chloride polymers and in the atmosphere that workers breathe be maintained at very low levels, it is necessary that the monomer content of the slurry and of the polymer recovered from it be sufficiently reduced that these requirements can be met.
Procedures for removing vinyl chloride from latexes of vinyl chloride polymers that have been produced by the emulsion polymerization of vinyl chloride have been reported, for example, in U.S. Pat. Nos. 2,662,867 (Hoertz) and 3,052,663 (Bodlaender et al.). These procedures cannot ordinarily be applied to the removal of vinyl chloride from aqueous slurries produced by the suspension polymerization technique for a number of reasons; e.g., they do not reduce the monomer contents of the slurries to the required very low level; they may cause foaming or otherwise interfere with the processing of the slurry; and they usually have a deleterious effect on the physical properties of the products.
The greater difficulty in removing vinyl chloride from aqueous slurries of vinyl chloride polymers produced by suspension polymerization than from latexes produced by emulsion polymerization of vinyl chloride is believed to result from differences in the particle size, particle structure, and molecular weights of the polymers and in the amounts of surfactants that they contain.
In aqueous slurries prepared by suspension polymerization processes, relatively low molecular weight vinyl chloride polymers are present as moderately-porous granules or agglomerates that range from about 25 microns to 500 microns in diameter, whereas latexes prepared by emulsion polymerization processes contain relatively high molecular weight vinyl chloride polymers as discrete, spherical, non-porous particles that have diameters in the range of about 0.01 micron to 2 microns. The surfactant content of the slurries is from 0.02% to 0.1% by weight; that of the latexes is from about 1% to 5% by weight. Because of the smaller size of the polymer particles, a given weight of emulsion-type polymer has a far greater surface area than the same weight of suspension-type polymer. These differences between typical suspension-type and emulsion-type vinyl chloride polymers are shown in the table that follows:
______________________________________ Vinyl Chloride Polymers Suspension-Type Emulsion-Type ______________________________________ Particle Size 25-500 0.01-2 (microns) Particle Structure Granules or Discrete agglomerates spherical particles Surface Area (m .sup.2 /g) 0.1-2.5 8-30 Molecular Weight Relatively Low Relatively High Porosity Moderately Non-porous porous Surfactant Content of Slurry or 0.02-0.1 1-5 Latex (%) ______________________________________
The greater surface area of the emulsion-type polymers results in faster diffusion of monomer from the polymer particles. The presence of larger amounts of surfactant in the latexes than in the slurries results in the recovery from them of polymers that have a higher level of impurities, which may degrade the electrical and other properties of the polymers.